The present invention relates to a method and apparatus for measuring the skin friction of foundation piles, which is for determining a dimensional and configurational specification for piles for supporting the intended structure.
The bearing capacity of foundation piles comprises two components, a point resistance and a skin friction. The point resistance represents the bearing capacity exhibited at the lower end of a pile placed in a ground against a force in the axial direction of the pile. The skin friction representing the bearing capacity against frictional resistance between soil or ground and the pile, along the pile shaft. In designing a structure to be built on foundation piles and to design the pile configuration economically and safely, it is extremely important to evaluate the point resistance and the skin friction, at the building site to appropriately evaluate of the bearing capacity of the foundation piles.
In the art of geotechnical engineering, how to appropriately evaluate the bearing capacity of piles is an important subject. There have been a variety of studies made, seeking to give a satisfactory solution to this subject. However, there has not yet been established a method which is relatively simple by which the bearing capacity of piles, particularly the skin friction thereof, can be determined with a high degree of accuracy.
In determining skin friction, it is generally practiced to determine the soil constant of the ground at the intended construction site and calculate the skin friction based on the determined soil constant. For determining soil constants, N values (blow counts found by the standard penetration test according to JIS A 1219) and qu values (unconfined compressive strength values found according to JIS A 1216) are employed. However, the N value represents soil characteristic parameter under a dynamic condition which is essential difference from skin friction which is a shear strength under a static condition. Thus, the qu value, represents the shear strength of soil under an undisturbed condition. However, when subjected to driving-in of a pile, the soil becomes disturbed. Its property undergoes a change.
Therefore, by any of today methods of determining the skin friction based on N values or qu values, it is infeasible to attain a satisfactory accuracy in the determination. Further, whereas in order to evaluate the skin friction accurately it is necessary to take into consideration each of the degree of disturbance which soil has undergone as a result of driving of a pile thereinto, the degree to which the soil restores its original condition as time lapses, and the influence on the friction resistance by a difference in the surface roughness of piles. These factors cannot be taken into account according to the evaluation methods making use of N values or qu values. Therefore, according to present evaluation methods, such methods often tends to result in under-evaluating skin friction and, in designing piles, placing more stress than necessary on safety.
A further known method of measuring the skin friction of piles comprises a loading test method according to ASTM D3966. Under this method, a load is applied on a pile driven in a ground. Determination is then made of displacement in the axial direction of the pile to evaluate skin friction. This test method can provide a bearing capacity value of the pile itself and is therefore advantageous in the light of the accuracy. However, such a load method involves the need of a large scale installation and is costly and time-consuming to operate. In practice it is extremely difficult to operate the load test method frequently. Generally, the load test method is terminated as soon as the design bearing capacity is reached. Thus, it is likely, even in the case of this test method, that the determination of the skin friction lacks accuracy. Thus, the design bearing capacity are usually conservatively set.